This invention relates generally to methods and apparatus for reconstruction of volumetric computed tomographic (CT) images, and more particularly to methods and apparatus for helical cone beam volumetric CT image reconstruction with three dimensional (3D) reconstruction view windowing to reduce or eliminate data redundancy.
In at least one known multi-detector row CT imaging systems, two-dimensional (2D) algorithms have been used to reconstruct tomographic images based on an approximation of cone beam (CB) geometry into fan beam (FB) geometry. As CB volumetric CT (VCT) technology becomes more prevalent, maintaining reconstruction accuracy has become more challenging. As a result of a significantly larger cone angle, CB-to-FB geometry approximations result in significant artifacts. To combat these artifacts, three dimensional (3D) reconstruction algorithms can be used in CB VCT. One such algorithm has been proposed by A. Katsevich in “Analysis of an exact inversion algorithm for spiral cone-beam CT,” Physics in Medicine & Biology vol. 47, pp. 2583–2598, 2002. A common feature of these helical CB reconstruction algorithms is the utilization of the view windowing function known as the “Tam window” to handle data redundancy. See, for example, U.S. Pat. No. 5,390,112 (assigned to General Electric) entitled “Three-dimensional computerized tomography scanning method and system for imaging large objects with smaller area detectors,” and P. E. Danielsson, P.-E., P. Edholm, J. Eriksson, and M. Magnusson-Seger, “Towards exact 3D-reconstruction for helical cone-beam scanning of long objects: A new arrangement and a new completeness condition,” International Meeting on Fully Three-dimensional Image Reconstruction in Radiology and Nuclear Medicine, Jun. 25–28, 1997.
More specifically, in a helical CB reconstruction, given a segment of a source trajectory, the data redundancy of a pixel in a field of view (FOV) that is in a plane orthogonal to the axis of the helical source trajectory, is location-dependent. Such a location-dependent data redundancy is non-uniform within an FOV, resulting in substantial shading and/or glaring artifacts in reconstructed images. Hence, handling data redundancy appropriately is one of the more challenging aspects of helical CB reconstruction. The most straightforward approach is to remove all data redundancy, such as by using Tam-windows.
Fundamentally, the Tam-window discards all redundant data by windowing a helical projection, because the projection data corresponding to an x-ray is exclusively within or outside the Tam-window. Theoretically, the analytic Tam-window is accurate in handling data redundancy in helical CB reconstruction. However, the implementation of the analytic Tam-window in the detector of a CB VCT scanner is actually a discrete sampling of the analytic one because of the finite dimension of detector elements. Thus, special care is needed to appropriately carry out such a discrete sampling. In fact, it has been found that a direct utilization of the Tam-window results in unexpected, substantial artifacts in tomographic images that have to be suppressed by extra compensation measures.